SMD rework station temperature not accurate !!!!!

[Shivram10]

Hi EEVBloggers,
I am Shivram from India, I am a beginner hobbyist, First of all I like to Thank Mr. Dave Jones for his amazing youtube videos and all the members of this forum I am learning so much things from this forum, I have been a regular visitor of this site, This is my first post,
My question is regarding SMD rework Station - my friend had recently bought a - 3in1 SMD Rework Station from DHGate.com for around US$129 its a Saike 909D model it has soldering, hot air gun and 15V DC variable power supply. He bought it 3-4 months back and few days back when I went to see the machine as I am planning to buy such SMD rework station in few months, we were testing the temperature of hot air gun and soldering station with my Vichy VC99 multimeter, I noticed that the temperature of the soldering iron is way off, he usually sets the temperature to 350^oCelsius, which when I measured with my multimeter it showed around 248-260^oCelsius. When I turned the max temp of the SMD station to 480^oC then it read not more than 360^oC. He cleans the solder iron tip with wet sponge and tins the iron tip regularly. When we opened the solder iron we saw that the tip of the heating element was turning black as circled in the images below (sorry for blurry images as we didn't had a digital camera at that moment it was from my cell phone camera)My friend works mostly on though hole components and occasionally on SMD components for through hole components the temperature is fine but he is facing difficulties in de-soldering SMD components as the solder iron is not hot enough.
We never used temperature controlled rework station before so we don't know if it is normal or not for the heating element to turn black.
The temperature of hot air gun is fairly accurate 10-15^oC less than what is set and the DC power supply is accurate.

Waiting for your replies

Thanks and Regards,
Shivram.

[hlavac]

I'd say that is normal with higher temperatures. My soldering iron certailny did turn a bit black with usage on the heating element area.
Isn't the temperature scale in degrees F or something?

[Shivram10]

I'd say that is normal with higher temperatures. My soldering iron certailny did turn a bit black with usage on the heating element area.
Isn't the temperature scale in degrees F or something?

@hlavac
Thanks for replying, the temperature is in Celsius not in Fahrenheit. It is mentioned on the manual of the Rework station.

[Shivram10]

Hi Shivram , i do not have your actual brand but a similar one .

There is a VR(variable resistor) on the board to set the temperature , mine is in the lower middle of the board for the iron , and left top-ish for the gun .

Set the front scale , then when reading the temp with the external sensor adjust the VR to read the same , its easy but it'll need a few trys to get it right (undershoot/overshoot) , mine was -40 or 60c out too and the gun only -5c so i left that alone .....

Re- that mark on the heater is nothing to worry about imo ...

@GEuser,
Thank you very much for your advice, The board on my friend's rework station is the exact same one, I first set the temp to 300^oC on the scale and then adjusted the VR several times and finally the temperature is now minus 10-15^oC of what I have set, which is fine for the work he does, I was worried about the black mark on the heating element which is as you and mr. hlavac have said is normal for high temperatures.

Once again Thanks a lot.

Regards,
Shivram